When I click to export, I am choosing "video and audio" and H.264 codec. I read the estimated size of the file in the export window and then when I export, the actual file is always about half the size.

Is it normal for FCPX to estimate 100MB on an exported file, and then deliver 50MB?

There always seems to be some headroom, the exported files use to be around 20% smaller. I recently exported a 3840 x 1650 clip, and the file indeed was almost half the size than what was predicted. I guess FCP calculates only with standardized dimensions.

If the larger master file quality looks about the same as the compressed version, then how does that compare to the playback quality in FCPX ? If it only looks worse on export that is somewhat unusual. Are you playing back the export full screen and possibly viewing it in the FCPX viewer in a much smaller window?

If the larger master file quality looks about the same as the compressed version, then how does that compare to the playback quality in FCPX ? If it only looks worse on export that is somewhat unusual. Are you playing back the export full screen and possibly viewing it in the FCPX viewer in a much smaller window?

Is it normal for FCPX to estimate 100MB on an exported file, and then deliver 50MB?

Yeah, H.264 exports are almost always turning out smaller than the estimation for me. Not half the size but at least significantly lower (15-40%). Seems ProRes estimates are much closer to the estimate though.